1. Field of the Invention
This invention relates to limiting reverse voltage swings at output stages of integrated circuits.
2. Prior Art
When a common emitter NPN driver transistor is formed in an epitaxial integrated circuit, there is a parasitic NPN formed whose emitter is the collector of the driver transistor, whose base is the P-type substrate, and whose collector is any other N-type epitaxial region on the integrated circuit. Thus, when the output of a circuit is an open collector NPN driver transistor, it is undesirable that external influences cause the voltage at the output to fall significantly below the ground (substrate) reference voltage. Such a decrease in output voltage can turn on the parasitic NPN transistor, injecting unwanted minority carriers into the substrate which will be collected by adjacent epitaxial regions. This collected current can be particularly disruptive if the adjacent epitaxial region is the base of a lateral PNP transistor or the collector of an NPN transistor. Clamping the output of the open collector NPN driver transistor to a voltage level close to the ground reference voltage prevents undesirable crosstalk due to minority carrier injection into the substrate.
A known way of overcoming this parasitic coupling of undesirable signals is to connect a low impedance diode between the emitter and collector of the output driver transistor so that the negative voltage swing is clamped by the diode and does not turn on the parasitic NPN transistor. However, a drawback to such a system is that the diode must be a large size to absorb typical output transients and it does not completely prevent the parasitic NPN transistor from turning on, but only reduces its current flow. Thus, to the extent that the parasitic NPN turns on, there is still undesired coupling from the driver transistor collector into adjacent and nonadjacent N-type epitaxial layer regions. Such regions typically include the collectors of other NPN transistors, the bases of other PNP transistors, and resistor tubs.
A particular problem caused by such parasitic current is that transistors adjacent to output transistors, which may be controlling other drivers, may turn on and cause an undesirable output. If the output transistors are connected to a motor, undesired actuation of various motor coils could cause motor stalling, hesitating or jerking.